Variable frequency tag

ABSTRACT

An antenna assembly is operative for receiving interrogating radiation at a variable frequency tag and generating a corresponding received signal, and for receiving a signature signal and radiating corresponding response radiation. A logic unit is operative for receiving the received signal and outputting the signature signal in response, the signature signal including a signature code for use in identifying the tag. A voltage controlled oscillator is operative for controlling a rate at which the signature code is output; and a power supply is operative for providing an electrical potential difference for energizing the tag. The voltage controlled oscillator is operable to output the signature code at a rate which is governed by the magnitude of the received signal.

The present invention relates to a variable frequency tag. Moreover, theinvention relates to a method of interrogating one or more tagsaccording to the invention. Furthermore, the invention also relates to atagging system incorporating one or more tags according to the inventionand operating according to the aforementioned method.

Tags capable of receiving interrogating radiation and emittingcorresponding response radiation are known. Such tags are often a fewmm's to cm's in size and designed to be affixed to products or to bepersonnel wearable. Moreover, such tags often include active electroniccircuits, for example one or more of amplifiers, microprocessors andsimple logic circuits. These electronic circuits require a supplypotential difference to operate. The supply potential can be providedfrom power sources incorporated into the tags, for example from one ormore button cells. Alternatively, the potential can be generated fromradiation received at the tags.

Generating the supply potential from received radiation results in thetags exhibiting a relatively limited operating range from associated taginterrogating devices, usually in the order of a few metres distance.There are often limits imposed to radiation electric field strengthsthat can be emitted from such interrogating devices, especially when thedevices operate at microwave frequencies in environments where personnelcan potentially be subjected to the radiation. Microwave radiation inthis context is defined as electromagnetic radiation having a frequencyin a range of 500 MHz to 90 GHz.

The inventor has appreciated that there are considerable benefits fromemploying tags which are not dependent on power sources local thereto.Tag operating lifetime can thereby be extended, tag construction can besimplified and further miniaturised, and tag manufacturing cost can bereduced. Such benefits are important where tags are employed to label amultitude of low cost products which are to be stored for long periodsof time, for example to mature, and then at the end of the periodindividually identified by remotely interrogating the tags.

The inventor has also appreciated that interrogating devices forinterrogating tags must be capable of coping with multiple tagcontention where several of the tags respond simultaneously tointerrogation. Conventional storage areas can accommodate many thousandsof individual products hence contention problems can be potentiallycomplex if such products are labeled with tags.

Thus, the inventor has appreciated that there is at least one problemassociated with conventional tags, namely:

-   (a) contention problems when a plurality of tags are simultaneously    interrogated;-   (b) limited operating range problems when the tags are powered by    radiation received thereat; and-   (c) limited tag lifetime when the tags are powered by power sources    local thereto.

In order to address one or more of the aforesaid problems, the inventorhas devised a new type of tag according to the invention.

According to a first aspect of the present invention, there is provideda variable frequency tag comprising:

-   (a) interfacing means for receiving interrogating radiation at the    tag and generating a corresponding received signal, and for    receiving a signature signal and radiating corresponding response    radiation;-   (b) processing means for receiving the received signal and    outputting the signature signal in response, the signature signal    including a signature code for use in identifying the tag;-   (c) clocking means for controlling a rate at which the signature    code is output; and-   (d) power supplying means for providing an electrical potential    difference for energizing the tag,    characterised in that the clocking means is operable to output the    signature code at a rate which is governed by the magnitude of the    received signal.

The tag provides the advantage that it is capable of addressing one ormore of the aforementioned problems, namely:

-   (a) resolving contention problems when a plurality of tags are    simultaneously interrogated;-   (b) extending operating range when the tags are powered by radiation    received thereat; and-   (c) extending tag lifetime when the tags are powered by power    sources local thereto.

The rate at which the signature code is output is defined to be one ormore of the following:

-   (a) the frequency at which the signature code itself is repetitively    output; and-   (b) the rate at which data bits comprising the signature code are    output each time the signature code is output.

Signature code is to be construed as any type of code, either analogueof digital or both, by which the tag can be identified.

Preferably, in order to circumvent a need for the tag to include its ownlocal power source or use its local power supply to a lesser extent, thesupplying means is coupled to the interfacing means, the supplying meansbeing operable to derive the potential difference from the receivedsignal. Thus, the tag can be powered by or its operation determined byradiation received thereat.

Received signals can often be of insufficient voltage amplitude to berectified to operate electronic circuits. Thus, preferably, thesupplying means includes a transformer for enhancing the potentialdifference applied to the clocking means and the processing means. It isespecially preferable that the transformer is a piezo-electrictransformer because such a transformer can be made compact and canprovide a suitable impedance at its connection ports for operating thetag.

In order to obtain a beneficial voltage amplitude increase to operatesemiconductor devices in the tag, the transformer preferably includes amultilayer primary region arranged to be driven by the received signal,and a single-layer secondary region at which the potential difference isgenerated, the primary and secondary regions being mechanically coupled.

The inventor has appreciated that the tag can be potentially damaged byexcess supply voltage when operated with relatively strong radiationreceived thereat. To address such potential damage, the supplying meanspreferably includes potential difference limiting means for preventingexcess supply potential damage to the processing means and the clockingmeans.

In practice, it is would especially desirable to use radio radiation tointerrogate the tag. Thus, conveniently, the interfacing means comprisesan antenna assembly operable to generate the response radiation from thereceived radiation by modulating reflectivity of the antenna assemblydepending upon tag power consumption. For example, the antenna assemblycan beneficially comprise a folded dipole antenna.

In order to assist with resolving contention and also to extend tagoperating range, it is especially preferable that the clocking means isoperable to clock the processing means at a rate which increases as thepotential difference increases. In many types of electronic circuit, forexample complementary metal-oxide-semiconductor (CMOS) circuits, powerconsumption increases with clocking rate. Thus, by reducing the clockingrate for relatively lower potential differences, tag power consumptionis reduced which is beneficial when the received signal is relativelyweak at greater operating distances.

In some tag interrogation systems, it is desirable that standardisedfrequencies are employed. To conform to such standardised frequencies,it is preferable that the clocking means is operable to increase therate at which the processing means is clocked in a stepwise manner inresponse to increase in the potential difference. More preferably, theclocking means comprises digital dividing means for dividing a masterclock signal to generate a clocking signal for clocking the processingmeans, the master clock signal being derived from the received signal;employing the received radiation to define the master clock signalprovides the benefit that tag operating frequency can be synchronouslylinked to interrogating devices interrogating the tag.

Alternatively, the clocking means preferably comprises digital dividingmeans for dividing a master clock signal generated by oscillating means,the master clock signal being substantially constant in operation.

In one embodiment of the invention, it is desirable that the clockingmeans is operable to increase the rate at which the processing means isclocked in a substantially linear manner in response to increase in thepotential difference. Alternatively, especially where greater tagoperating distances are anticipated, it is preferable that the clockingmeans is operable to increase the rate at which the processing means isclocking in a substantially logarithmic manner in response to increasein the potential difference.

Low cost is an important attribute of the tag. The inventor hasappreciated that the clocking means can be implemented in a simpleinexpensive manner by connecting a plurality of serially connected logicgates together with feedback therearound for generating a clockingsignal for clocking the processing means, the logic gates having asignal propagation therethrough which is a function of the potentialdifference, thereby rendering the clocking signal frequency dependent onthe potential difference. It is found especially preferable for theoscillator to comprise a ring-of-three logic gates configured withfeedback therearound for generating the clocking signal.

When designing the tag, the inventor has found is preferable for theprocessing means to account for a majority of power required to operatethe tag; such power consumption is determined by the clocking rate andcan be used to provide the tag with its extended range. More preferably,the processing means includes CMOS logic circuits for generating thesignature code, the logic circuits operable to consume increasing powerin operation as their clocking rate is increased.

Contention arises when a plurality of the tags respond simultaneously.In order to give each tag an opportunity to provide an uninterruptedresponse, the processing means is preferably operable to output thesignature code repetitively with pause intervals therebetween duringwhich the code is not output.

The inventor has appreciated that there are other ways also of resolvingcontention. Preferably, therefore, the processing means is receptive toone or more synchronisation pulses in the received signal and isswitchable to a temporary wait state in which the processing means doesnot output its signature code when the one or more synchronisationpulses do not align to a synchronisation time window after the tagoutputs its signature code. Use of such pulses and associated timewindow enables competing tags to be temporarily disabled for allowingindividually uninterrupted responses from each of the tags to bereceived.

In a second aspect of the present invention, there is provided aninterrogating device for interrogating one or more tags according to thefirst aspect of the invention, the device characterised in that itincorporates:

-   (a) signal generating means for generating an interrogating signal;-   (b) interrogation interfacing means for radiating the interrogating    signal as interrogating radiation towards said one or more tags, and    for receiving response radiation from said one or more tags and    generating a corresponding response signal; and-   (c) signal processing means for filtering the response signal and    thereby isolating signal spectral components from each of said one    or more tags and extracting signature codes from the signal    components for identifying said one or more tags.

On account of the variable frequency nature of the tag according to thefirst aspect of the invention, the device exploits such nature toresolve contention between a plurality of tags respondingsimultaneously.

When one or more tags are operated such that their clocking frequency isdetermined by the amplitude of received signals, the rates at which thesignature codes are output are a function of the received signalamplitudes and hence tag distances from the interrogating device. Thus,by moving a plurality of the tags spatially with respect to the device,a variety of clocking rates for each tag can be received by the deviceand used to resolve multiple tag contention. Thus, preferably, thedevice includes tag transporting means for transporting in operationsaid one or more tags spatially in relation to the interrogationinterfacing means, the signal processing means being operable to samplethe response signal repetitively at intervals for resolving multiple tagcontention.

As an alternative to spatially moving said one or more tags, the deviceitself can be spatially repositioned. Such repositioning can beconveniently achieved by arranging for the interrogation interfacingmeans to comprise a plurality of antennae spatially disposed in relationto said one or more tags for radiating the interrogating radiation, thesignal processing means operable to switch in sequence through theantennae to interrogate said one or more tags from varying distances,and to process corresponding response signals at the device forresolving multiple tag contention.

In a third aspect of the present invention, there is provided a taggingsystem incorporating one or more tags according to the first aspect ofthe invention and an interrogating device according to the second aspectof the invention for interrogating and identifying said one or moretags.

In a fourth aspect of the present invention, there is provided a methodof interrogating a variable frequency tag using an interrogating device,the method characterised in that it includes the steps of:

-   (a) emitting interrogating radiation from the device towards the    tag;-   (b) receiving the interrogating radiation at the tag and generating    a corresponding received signal;-   (c) receiving the received signal at processing means of the tag;-   (d) outputting a signature signal from the processing means in    response to receiving the received signal thereat, the signature    signal including a signature code for use in identifying the tag,    the signature code being output at a rate dependent upon a supply    potential difference energizing the tag;-   (e) radiating the signature signal as response radiation from the    tag;-   (f) receiving the response radiation from the tag at the device and    generating a corresponding interrogation received signal thereat;-   (g) filtering the interrogation received signal in the device to    isolate one or more spectral components corresponding to the tag,    extracting the signature code of the tag from said one or more    spectral components and then correlating said signature code with    one or more signature templates to identify the tag.

Preferably in the method, the supply potential difference is derivedfrom the received signal to circumvent a need to include a local powersource in the tag. More preferably, the supply potential difference isenhanced by using a piezo-electric step-up transformer to provide thetag with greater operating range.

In a fifth aspect of the present invention, there is provided a methodof resolving contention between a plurality of variable frequency tagsinterrogated from an interrogating device, the method characterised inthat it includes the steps of:

-   (a) emitting interrogating radiation from the device towards the    tags;-   (b) receiving the interrogating radiation at each tag and generating    a corresponding received signal thereat;-   (c) receiving at each tag the received signal at processing means of    the tag;-   (d) outputting a signature signal from the processing means of each    tag in response to receiving the received signal thereat, the    signature signal including an associated signature code for use in    identifying the tag, the signature code being output at a rate    dependent upon a supply potential difference energizing the tag, the    potential difference being derived from the received signal of the    tag;-   (e) radiating the signature signal of each tag as response radiation    from the tag;-   (f) receiving the response radiation from the tags at the device and    generating a corresponding interrogation received signal thereat;-   (g) filtering the interrogation received signal at the device to    isolate one or more spectral components corresponding to the tags,    extracting the signature codes of the tags from said one or more    spectral components and then correlating said signature codes with    one or more signature templates for identifying the tags; and-   (h) if contention exists with respect to one or more of the    components, repetitively modifying a spatial relationship between    the device and the tags and repeating steps (a) to (g) until the    contention is resolved.

In a sixth aspect of the present invention, there is provided a methodof resolving contention between a plurality of variable frequency tagsinterrogated from an interrogating device, the method characterised inthat it includes the steps of:

-   (a) emitting interrogating radiation from the device towards the    tags;-   (b) receiving the interrogating radiation at each tag and generating    a corresponding received signal thereat;-   (c) receiving at each tag the received signal at processing means of    the tag;-   (d) outputting a signature signal from the processing means of each    tag in response to receiving the received signal thereat, the    signature signal including an associated signature code for use in    identifying the tag, the signature code being output at a rate    dependent upon a supply potential difference energizing the tag, the    potential difference being derived from the received signal of the    tag, the signature code being repetitively output with pauses    therebetween during which the code is not output;-   (e) radiating the signature signal of each tag as response radiation    from the tag;-   (f) receiving the response radiation from the tags at the device and    generating a corresponding interrogation received signal thereat;-   (g) filtering the interrogation received signal at the device to    isolate one or more spectral components corresponding to the tags,    extracting the signature codes of the tags from said one or more    spectral components and then correlating said signature codes with    one or more signature templates for identifying the tags; and-   (h) if contention exists with respect to one or more of the    components, repeating steps (a) to (g) until the contention is    resolved.

In a seventh aspect of the present invention, there is provided a methodof resolving contention between a plurality of variable frequency tagsinterrogated from an interrogating device, the method characterised inthat it includes the steps of:

-   (a) emitting interrogating radiation from the device towards the    tags;-   (b) receiving the interrogating radiation at each tag and generating    a corresponding received signal thereat;-   (c) receiving at each tag the received signal at processing means of    the tag;-   (d) identifying one or more pulses present in the received signal at    each tag, outputting an associated signature signal from the    processing means of the tag in response to receiving the received    signal thereat depending on whether or not said one or more pulses    are coincident with a time window associated with the tag, the    signature signal including an associated signature code for use in    identifying the tag, the signature code output at a rate dependent    upon a supply potential difference energizing the tag, the potential    difference being derived from the received signal of the tag;-   (e) radiating the signature signals as response radiation from one    or more of the tags;-   (f) receiving the response radiation from said one or more of the    tags at the device and generating a corresponding interrogation    received signal thereat;-   (g) filtering the interrogation received signal at the device to    isolate one or more spectral components corresponding to said one or    more of the tags, extracting the signature codes of said one or more    of the tags from said one or more spectral components and then    correlating said signature codes with one or more signature    templates for identifying said one or more of the tags; and-   (h) if contention exists with respect to one or more of the    components, outputting said one or more pulses in the interrogating    radiation to temporarily disable one or more of the tags from    responding and repeating steps (a) to (g) until the contention is    resolved.

Preferably, the time window of each tag is temporally dependent upon aclocking rate at which the processing means of the tag is clocked, theclocking rate in turn being dependent upon the supply potentialdifference of the tag. The method provides the advantage that theinterrogating device can control one or more of the tags individually.

Embodiments of the invention will now be described, by way of exampleonly, with reference to the following diagrams in which:

FIG. 1 is a schematic block diagram of principal sections of a variablefrequency tag according to the invention;

FIG. 2 is a circuit diagram illustrating a first practicalimplementation of the tag shown in FIG. 1 employing direct power supplypotential loading;

FIG. 3 is a circuit diagram illustrating a first alternativeimplementation of the tag shown in FIG. 2 employing direct tag inputloading;

FIG. 4 is a circuit diagram illustrating a second alternativeimplementation of the tag shown in FIG. 2 employing oscillator frequencymodulation loading;

FIG. 5 is a circuit diagram illustrating a third alternativeimplementation of the tag shown in FIG. 2 employing a auxiliaryload-modulated antenna for providing reflected radiation;

FIG. 6 is a circuit diagram illustrating a fourth alternativeimplementation of the tag shown on FIG. 2 employing dual oscillators;

FIG. 7 is an illustration of an interrogating device according to theinvention usable for interrogating a plurality of tags of the type shownin FIGS. 2, 3, 4, 5, 6;

FIG. 8 is a graph illustrating a spectral response of radiation receivedby the device shown in FIG. 7;

FIG. 9 is an illustration of a modified version of the interrogatingdevice shown in FIG. 7, the modified version including multipletransmitter antennae for resolving contention problems when a pluralityof tags as illustrated in FIGS. 1 to 6 are interrogated simultaneously;

FIG. 10 is a circuit diagram of a modified version of the tagillustrated in FIG. 2 including a piezo-electric transformer forproviding the tag with extended operating range when powered byradiation received thereat;

FIG. 11 is a circuit diagram of a modified version of the tagillustrated in FIG. 10 including direct load modulation to an input ofthe tag; and

FIG. 12 is a schematic diagram of an alternative interrogating deviceincluding dual orthogonally disposed loop antennae, the deviceinterrogating a tag as shown in FIG. 2 equipped with a loop antennaappropriate for receiving radiation from the alternative interrogatingdevice.

Referring to FIG. 1, there is shown principal sections of a variablefrequency tag according to the invention; the tag is indicated generallyby 10. The tag 10 comprises a plurality of interconnected sections,namely an antenna assembly 12, an impedance matching network 14, a diodedetection assembly 16, a voltage controlled oscillator 18 and a logicunit 20. The logic unit 20 can be implemented as a state machine, forexample using ROM and associated logic gates; alternatively, the logicunit 20 can be implemented by using a microcontroller device. The logicunit 20 incorporates complementary metal oxide semiconductor (CMOS)devices whose current consumption increases as their clocking rate isincreased.

Interconnection of the sections of the tag 10 will now be described.

The antenna assembly 12 is coupled via the matching network 14 to afirst port of the diode detection assembly 16. The detection assembly 16comprises a second port which is connected to a power input of thevoltage controlled oscillator 18 and also to a power input of the logicunit 20. A clock signal CLK output of the oscillator 18 is coupled to aclock input of the logic unit 20.

Operation of the tag 10 will now be described.

The tag 10 receives interrogating radiation 22 at the antenna assembly12 and generates a corresponding received signal S1. The interrogatingradiation 22 can, for example, be microwave radiation having a carrierfrequency in the order of 2.5 GHz, namely in a range 500 MHz to 90 GHz.Alternatively, the radiation 22 can be high frequency radiation having acarrier frequency in the order of 13.56 MHz, namely in a range of 5 MHzto 100 MHz. As a further alternative, the radiation 22 can be lowfrequency radiation having a carrier frequency in the order of 125 kHz,namely in a range of 20 kHz to 500 kHz where inductive coupling effectsare significant.

The signal S1 passes through the network 14 to the diode assembly 16whereat it is rectified to provide a substantially unipolar butfluctuating potential difference P which is applied to the voltagecontrolled oscillator 18 and to the logic unit 20. The oscillator 18generates a clock signal CLK whose frequency F is a function of thepotential difference P; the frequency F increases as the potentialdifference P increases. Preferably, the frequency F is substantiallylinearly related, for example to within a linearity deviation of 20%, tothe potential difference P. Alternatively, the frequency F can be madeto vary in a non-linear manner, for example in a substantiallylogarithmic manner, in relation to changes in the potential differenceP.

The logic unit 20 is clocked by the clock signal CLK and is designed toprovide a variable power load to the diode assembly 16, for example byway of an input/output terminal of the logic unit 20 being coupled tothe second port of the diode assembly 16, thereby modulating thepotential difference P. The logic unit 20 is arranged to modulate thedifference P in a temporal manner depending upon a signature codeprogrammed into the unit 20 which uniquely distinguishes it from othersimilar tags. The rate at which the logic unit 20 modulates thedifference P is determined by the frequency of the clock signal CLK.Moreover, as elucidated in the foregoing, power consumption of the logicunit 20, which accounts for majority of power dissipated within the tag10, increases as the frequency of the clock signal CLK increases. Thus,when the tag 10 is operated at a relatively longer distance from a taginterrogating device, the oscillator 18 outputs the clock signal CLK ata relatively lower frequency and the logic unit 20 consumes relativelyless power. Conversely, when the tag 10 is operated at a relativelyshorter distance from the interrogating device, the oscillator 18outputs the clock signal CLK at a relatively higher frequency and thelogic unit 20 consumes relatively more power; the tag 10 thereby is alsocapable of responding faster back to the interrogating device. Thus, bydeliberately reducing the frequency of the clock signal CLK when the tag10 is operating at a relatively greater distance from the interrogatingdevice, the tag 10 is thereby provided with enhanced operating rangecompared to a conventional tag whose operating clock frequency ismaintained substantially constant irrespective of operating distancefrom an associated interrogating device. The interrogating devicedetects interrogating radiation received at the tag 10 which isreflected therefrom and received back at the interrogating device;temporal variations in the amount of radiation reflected is determinedpredominantly by instantaneous power consumed by the logic unit 20.

The inventors have appreciated that design of the tag 10 is not trivialand several design aspects have to be taken into consideration, namely:

-   (a) the diode assembly 16 exhibits an input impedance at its first    port which is a function of its video resistance which, in turn, is    a function of its output current delivered at the second port;-   (b) the potential difference P provided at the second port of the    diode assembly 16 is a function of output current delivered from the    second port on account of assembly 16 exhibiting a finite output    resistance; and-   (c) the logic unit 20, by way of its CMOS-type construction,    exhibits a power consumption which is substantially a function of    the square of the potential difference P and also substantially    linearly or logarithmically related, at a given potential difference    P, to the frequency of the signal CLK and tag 10 circuit capacitance    C which is substantially constant.

Thus, power consumption Q of the tag 10 can be determined to a firstorder from Equation 1 (Eq. 1):

$\begin{matrix}{Q = {{kF}( {\frac{1}{2}{CP}^{2}} )}} & {{Eq}.\mspace{14mu} 1}\end{matrix}$

-   where-   k=a proportionality constant; and-   F=f (P) where f is a function, for example a substantially linear or    logarithmic function.

It will be appreciated that several relationships, as defined by theaforesaid function f, between the potential difference P and thefrequency of the signal CLK are possible depending upon design of thevoltage controlled oscillator 18.

There are a number of benefits arising from operating the tag 10 with avariable frequency clock rate, namely:

-   (a) the tag 10 can be provided with an extended spatial operating    range whilst still responding to an associated interrogating device;    and-   (b) the tag 10 provides a response which is frequency shifted    depending upon operating distance which assists to resolve    contention issues when several tags of similar'design to the tag 10    are interrogated simultaneously.

The sections of the tag 10 schematically illustrated in FIG. 1 will nowbe described in further detail with reference to FIG. 2.

The antenna assembly 12 comprises a folded dipole antenna for receivingthe radiation 22 and generating the signal S1 between first and secondterminals of the assembly 12. The impedance matching network 14 isimplemented by a blocking capacitor 24. The diode detection assembly 16is implemented by a dual-zero-bias Schottky diode detector 26 comprisinga pair of serially connected Schottky diodes and a bypass capacitor 28.Moreover, the logic unit 20 is implemented as a CMOS microcontrollertype PIC12C509ESA manufactured by Microchip Technology Inc., a companybased in the USA.

The voltage controlled oscillator 18 is implemented as a “ring-of-three”oscillator comprising a series of first, second and third invertinglogic gates 30 a, 30 b, 30 c respectively connected in a feedback loopconfiguration as illustrated. Moreover, the oscillator 18 is preferablyimplemented using a standard proprietary 74HC04 CMOS device.Furthermore, the “ring-of-three” oscillator oscillates at a frequencydetermined by the propagation delay of its inverting logic gates whichis strongly dependent upon the potential difference P generated by thediode assembly 16.

Interconnection of the sections of the tag 10 will now be described withreference to FIG. 2.

The first terminal of the antenna assembly 12 is connected to a firstelectrode of the blocking capacitor 24. A second electrode of thecapacitor 24 is coupled to a mid-anode-cathode terminal of the dualSchottky detector 26 as illustrated. A cathode electrode of the detector26 is connected to a first electrode of the bypass capacitor 28, to aninput/output (I/O) terminal and a positive supply VDD terminal of themicrocontroller and also to a positive supply VDD terminal of theaforesaid 74HC04 device. An anode electrode of the detector 26 iscoupled to the second terminal of the antenna assembly 12, to a secondelectrode of the bypass capacitor 28 and to ground VSS terminals of themicrocontroller and the 74HC04 device. The 74HC04 device comprises athird terminal whereat the clock signal CLK is output, the thirdterminal coupled to a microcontroller clock input terminal of themicrocontroller.

Operation of the tag 10 will now be described with reference to FIG. 2.

An interrogating device (not shown in FIG. 2) emits the interrogatingradiation 22 which propagates to the tag 10 where it is received at theantenna assembly 12. The assembly 12 converts the radiation 22 receivedthereat into the signal S1 which is coupled through the blockingcapacitor 24 to the dual Schottky detector 26 which rectifies the signalS1 to generate a unipolar potential, namely the potential difference P,across the bypass capacitor 28. The potential difference P powers the74HC04 device which oscillates and outputs the clock signal CLK to themicrocontroller. The difference P also powers the microcontroller intooperation.

The microcontroller is pre-programmed with a signature code which isunique for the tag 10. The microcontroller switches its input/outputterminal I/O in a periodic manner governed by the signature code toexhibit a temporally varying load across the bypass capacitor 28.Variations in the load in turn influence an impedance presented by thedual Schottky detector 26 via the blocking capacitor 24 to the antennaassembly 12 and thereby a proportion of the radiation 22 which isreflected from the antenna assembly 12 back to the interrogating device.The interrogating device is receptive to radiation reflected backthereto, converts this reflected radiation to a signal which is thenprocessed to yield the temporal fluctuations and hence thepre-programmed signature code of the tag 10. The interrogating device istherefore capable of determining presence of the tag 10 and its specificidentity from its signature code.

It will be appreciated that the tag 10 illustrated in FIG. 2 can bemodified without departing from the scope of the invention. For example,the dual Schottky detector 26, the 74HC04 device and the microcontrollercan preferably be implemented using a single application specificintegrated circuit (ASIC); such an approach limits the number of partsneeded to be assembled to manufacture the tag 10 and thereby reduces itsfabrication cost. Moreover, the antenna assembly 12 can be preferablyinexpensively implemented using etched or printed metal film conductors.

The tag 10 can be encapsulated to protect and isolate it from itsenvirons. Such encapsulation is important where, for example, the tag 10is used to label items of clothing in dry cleaning industries. In theseindustries, items of clothing are subject to one or more of elevatedtemperature and copious quantities of organic solvent; the items have tobe uniquely identifiable for returning to their respective clients aftercleaning. The microcontroller preferably includes non-volatile memory,for example an E²PROM, so that it can be programmed with a uniquesignature code; this enables the tag 10 to be mass-produced and thencustomized post-production.

It will be appreciated that the tag 10 can be modified to include alocal source of power, for example a lithium button cell. The modifiedtag would retain the characteristic that its voltage controlledoscillator 18 outputs its clock signal CLK at a frequency which isdependent on a supply potential provided by the aforesaid button cell.Operating lifetime of the modified tag can thereby be extended byreducing power consumed from the cell when expiration of the cell isimminent, namely when its internal resistance is tending to increase.

It will also be appreciated that the microcontroller can have one ormore sensors attached thereto for sensing environs of the tag 10. Suchsensors can, for example, be one or more of temperature sensors, gassensors and biosensors so that the tag 10 can be used to remotelymonitor the condition or maturity of products which the tags are used touniquely identify. In this respect, the tag 10 can beneficially be usedin the brewing and food processing industries. Moreover, where thesensors are indicative of wear, the tag 10 can be used to signal when apart associated therewith is worn and needs replacing, for example in anautomotive application.

In FIG. 3, a first alternative version of the tag 10 is indicated by 50.The first alternative tag 50 is similar to the tag 10 except that theinput/output (I/O) terminal is connected via a load resistor 52 to thesecond electrode of the capacitor 24. In operation, the microcontrolleremploys the input/output (I/O) terminal to modulate a potentialdeveloped across the load resistor 52 and thereby apply load modulationto the antenna 12 via the capacitor 24. By applying such loadmodulation, a proportion of the radiation 22 reflected from the tag 50is correspondingly modulated.

The first alternative tag 50 provides the benefit that it can loadmodulate the radiation 22 at a greater frequency than is possible in thetag 10. The greater modulation frequency arises because themicrocontroller in the first alternative tag 50 does not need to varyits potential difference P which is buffered by the capacitor 28. Thus,the first alternative tag 50 is capable of responding at an enhanceddata rate to the interrogating radiation 22 compared to the tag 10.

In FIG. 4, a second alternative version of the tag 10 is indicated by60. The second alternative tag 60 is similar to the tag 10 except thatthe oscillator 18 further incorporates a first resistor 62, a secondresistor 64, a blocking capacitor 66 and a varicap diode 68. The first,second and third logic inverter gates 30 a, 30 b, 30 c respectively areconnected in series starting with the first logic gate 30 a and endingwith the third logic gate 30 c. An output from the third gate 30 c isconnected to the clock input of the microcontroller and via the firstresistor 62 to an input of the first gate 30 a and also via thecapacitor 66 to a cathode electrode of the varicap diode 68. The cathodeelectrode is coupled through the second resistor 64 to the input/output(I/O) terminal of the microcontroller.

In operation, the second alternative tag 60 receives the radiation 22 atits antenna assembly 12 and generates the corresponding received signalS1. The signal S1 is coupled through the capacitor 24 to the detectionassembly 16 which, in turn, rectifies the signal S1 to generate thepotential difference P. The potential difference P energises theoscillator 18 and also the microcontroller. The oscillator 18 generatesthe clock signal CLK which clocks the microcontroller. Themicrocontroller periodically outputs its signature code at itsinput/output (I/O) terminal which modulates capacitance exhibited by thevaricap diode 68 and hence modulates the frequency at which theoscillator 18 oscillates. As power consumption of the tag 60, arisingprincipally within the microcontroller, varies in response to theoscillating frequency of the oscillator, a varying load modulated by thesignature code is presented to the detection assembly 16 which, in turn,modulates a proportion of the radiation 22 reflected from the tag 60.

In FIG. 5, a third alternative version of the tag 10 is indicated by 80.The third alternative tag 80 is identical to the tag 10 except that itfurther includes an auxiliary dipole antenna 82 comprising first andsecond dipole patches 84 a, 84 b mutually coupled via a pin diode 86.The second dipole patch 84 b is connected to a cathode electrode of thediode 86 and also to the second terminal of the antenna assembly 12. Thefirst dipole patch 84 a is coupled to an anode electrode of the pindiode 86 and also via a bias resistor 88 to the input/output (I/O)terminal of the microcontroller.

The third alternative tag 80 operates in an identical manner to the tag10 except that the microcontroller is operable to modulate a currentflowing through the pin diode 86 and thereby modulate its dynamicresistivity and hence modulate a proportion of the radiation 22 receivedat the auxiliary antenna 82 which is reflected back.

It will be appreciated that the third alternative tag 80 can be furthermodified by replacing the pin diode 86 with a gallium arsenide fieldeffect transistor (GaAs-FET) configured to function as a negativeresistance and thereby provide an enhanced degree of reflected radiationfrom the auxiliary antenna 82. Moreover, the antenna assembly 12 and theauxiliary antenna 82 can each be tuned to different radiationfrequencies so that when the third alternative tag 80 is interrogated bythe radiation comprising first and second radiation components, thefirst component couples efficiently through the antenna assembly 12 andenergizing the tag 80 and the second component couples efficiently intothe auxiliary antenna 82 and is load modulated therein. The third tag 80also provides the benefit that reflected radiation from the tag 80 canbe modulated more rapidly than the tag 10.

The third tag 80 can be further modified by including a surface acousticwave (SAW) oscillator comprising a SAW resonator and an associated gaincomponent such as transistor. In such a further modified tag, the SAWoscillator can be energized by the potential difference P and gated fromthe microcontroller, in a manner conveying the signature code, with arelatively low duty cycle, for example powered for 0.5 to 5% of thetime, to oscillate and thereby generate a signal for emission asreflected radiation from the auxiliary antenna 82. Such a furthermodified tag provides the advantage that it can respond back at afrequency different to the frequency of the radiation 22 used toenergize the further modified tag.

The third tag 80 can be additionally modified so that themicrocontroller includes an E²PROM, for example for providingnon-volatile memory for storing the tag's signature code. Themicrocontroller can be arranged to deliberately slow down the oscillator18 to allow a greater potential difference P to develop prior toperforming an energy-intensive function, for example reprogramming theE²PROM with a new signature code.

In FIG. 6, a fourth alternative version of the tag 10 is indicated by100. The fourth alternative tag 100 is identical to the tag 10 exceptthat it further includes a logic oscillator 102 coupled to themicrocontroller clock input. Moreover, the oscillator 18 is coupled to agating input of the microcontroller. Furthermore, the input/output (I/O)terminal of the microcontroller is coupled via a load resistor 104 tothe detection assembly 16 to load modulate it directly as elucidated inthe foregoing.

In operation, the fourth tag 100 receives the radiation 22 which iscoupled via the antenna assembly 12, the capacitor 24 and the detectionassembly 16 to generate the potential difference P. The potential Penergizes the oscillators 18, 102 and also the microcontroller. Thelogic oscillator 102 is designed to oscillate at a substantiallyconstant frequency irrespective of fluctuations in the potential P andhence clock the controller at a substantially constant rate. Incontrast, the oscillator 18 oscillates at a frequency which is highlydependent on the potential P, for example substantially in a linear orlogarithmic manner. The logic oscillator 102 ensures that themicrocontroller operates at a stable frequency and therefore outputs itssignature code at a predictable rate. The microcontroller employs theclock signal that it receives from the oscillator 18 to determine howfrequently it repeats the signature code. The fourth tag 100 providesthe benefit that the signature code is output at a predictable ratethereby easing signal processing operations at devices interrogating thefourth tag 100. As a result of the frequency of repetition of the codebeing dependent on the potential difference P and the fact that thefourth tag 100 spends most of its active time idling, for example 90% ormore, between outputting its signature code, contention is circumventedbecause a plurality of the fourth tags 100 operating concurrently willbe outputting their signature codes asynchronously; there will beinstances when only one of the plurality of tags is outputting itssignature code which can therefore be unambiguously identified byinterrogating devices interrogating the plurality of fourth tags.

In practice, tags according to the design of the tags 10, 50, 60, 80,100 can be manufactured in large numbers, for example in millions, inspecially adapted production machinery. In use, many such tags willoften simultaneously be within range of the aforementioned interrogatingdevice. It is an important aspect of the interrogating device that itcan cope with contention between tags when attempting to simultaneouslyinterrogate a plurality thereof. Coping with such contention is animportant practical aspect of any commercial tagging system.

Referring next to FIG. 7, there is shown an interrogation deviceaccording to the invention usable for interrogating a plurality of tags10 a, 10 b of similar design to the tag 10 illustrated in FIGS. 1 and 2;the interrogating device is indicated generally by 200 and shownincluded within a dashed frame 210. The interrogating device 200comprises a radio frequency oscillator 220, a power amplifier 230, atransmitter antenna 240, a receiver antenna 250, a receiver amplifier260, a single side-band mixer 270, and a baseband receiver 280 includingdigital signal processors and digital filters.

Interconnection within the interrogating device 200 will now bedescribed.

The oscillator 220 includes a radio frequency signal S_(RF) output, thesignal S_(RF) being matched to a radiation frequency range over whichthe tag 10 is responsive. The signal S_(RF) output is coupled to asignal input of the power amplifier 230 and to a first input port of themixer 270. An output from the power amplifier 230 is coupled to thetransmitter antenna 240. Likewise, the receiver antenna 250 is connectedto an input port of the receiver amplifier 260. An output of thereceiver 260 is coupled to a second input port of the mixer 270. Anoutput port of the mixer 270 is connected to a signal input of thebaseband receiver 280.

Operation of the device 200 interrogating the tags 10 a, 10 b will nowbe described with reference to FIG. 7. The tags 10 a, 10 b are spatiallydistributed at distances of L1, L2 respectively from the transmitterantenna 240.

The oscillator 220 generates the signal S_(RF). The signal S_(RF)propagates to the amplifier 230 and to the first port of the mixer 270.The amplifier 230 amplifies the signal S_(RF) to produce a correspondingamplified signal AS_(RF) which passes to the transmitter antenna 240wherefrom it is radiated as the radiation 70 towards the tags 10 a, 10b. The radiation propagates to the tags 10 a, 10 b to generate receivedsignals S1 therein that are rectified to generate corresponding supplypotential differences P in the tags 10 a, 10 b for energising them. Thetags 10 a, 10 b function as described in the foregoing with the tags 10a, 10 b having their associated voltage controlled oscillators 50operating at clocking frequencies of F1 and F2 respectively. In FIG. 7,the distance L1 is shorter than the distance L2 which results in thefrequency F1 being greater than the frequency F2. In other words, thetag 10 a is clocked at a greater rate than the tag 10 b because the tag10 a is relatively nearer the transmitter antenna 240 than the tag 10 b.

The tags 10 a, 10 b modulate their respective radiation reflectivitiesto provide reflected radiation 300, 310 respectively. The radiation 300,310 propagates from the tags 10 a, 10 b respectively to the receiverantenna 250 at which the radiation 300, 310 is converted into acorresponding received signal S_(TF). The signal S_(TF) propagates tothe receiver amplifier 260 in which it is amplified to generate acorresponding amplified signal AS_(TF) which passes to the second inputport of the mixer 270. The mixer 270 then mixes, namely multipliestogether to generate product components, the signals AS_(TF), S_(RF) togenerate a baseband signal S_(BB) which then propagates to the basebandreceiver 280 to undergo further signal processing which will bedescribed later.

Referring now to FIG. 8, there is shown a graph illustrating a spectralresponse of the signal S_(BB). The graph includes an abscissa axis 400indicating frequency increasing from left to right, and an ordinate axis410 indicating increasing signal amplitude from bottom to top. Thesignal S_(BB) comprises a first sideband component 420 arising from thetag 10 b and also a second sideband component 430 arising from the tag10 a. As a consequence of the tag 10 b being at a greater distance fromthe transmitter antenna 240 than the tag 10 a, the first sidebandcomponent 420 is at a relatively lower frequency than the component 430in the spectral response.

The components 420, 430 include information concerning the signaturecodes of the tags 10 b, 10 a respectively; moreover, the components 420,430 can include additional information from the tags 10 b, 10 a, forexample sensor measurements from sensors connected to or incorporatedinto the tags 10 b, 10 a. The receiver 280 isolates the components 420,430 by using frequency-selective digital filters and then extracts thesignature codes therefrom. Techniques for performing such filtration andextraction are known in the art of electronic circuit design.

The tags 10 a, 10 b can be designed to modulate their respectiveradiation reflectivities in the form of Manchester bi-phase encodeddata; when encoding data using Manchester bi-phase techniques, data andclock signals are exclusively OR-ed to generate corresponding Manchesterbi-phase encoded data. Alternative forms of modulation which can beemployed include FM0 and FM1 formats, FM0 and FM1 being generally knownwithin the technical field of radio communication systems.

When many tags similar in design to the tag 10 are interrogatedsimultaneously from the device 200, an ambiguous contentious situationcan arise where two or more tags are equidistant from the transmitterantenna 240. When such contention arises, spectral components from thetags processed by the receiver 280 will overlap and will beuninterpretable. The inventor has appreciated that there are a number ofways in which the device 200 and the tags 10 a, 10 b can be operated toaddress the contention.

In a first approach, the tags 10 a, 10 b are designed such that theirrespective microcontrollers do not continuously modulate radiationreflected from their tags but for only a fraction of the time duringwhich they are interrogated. As the clocking frequencies of the tags 10a, 10 b will be slightly different in practice, they will functionasynchronously such that at certain times they will be modulatingreflected radiation alternately. The receiver 280 will observe suchalternation as a single component in the spectral response whosesignature code is alternating. Processors in the receiver 280 can beprogrammed to identify such alternation of signature code and interpretit to denote the presence of more than one responding tag equidistantfrom the transmitter antenna 240.

In a second approach, the device 200 illustrated in FIG. 7 is modifiedinto an alternative interrogating device as shown in FIG. 9; thealternative device is indicated generally by 500. The device 500includes three transmitter antennae assemblies TX1, TX2, TX3 connectedto switching terminals a, b, c respectively of a switch unit 520. Awiper terminal d of the switch unit 520 is connected to the output ofthe power amplifier 230. The baseband receiver 280 is also modified toprovide an antenna select signal S_(ANT) which is coupled to the switchunit 520 whereat it is used to control connection of the wiper terminald to a preferred one of the aforementioned three switch terminals a, b,c.

The alternative device 500 additionally includes a movable conveyor belt510 onto which products and their associated attached tags 10 a, 10 bare transported relative to the antennae assemblies TX1, TX2, TX3.Distances from the antennae assemblies TX1, TX2, TX3 are denoted in FIG.9 by L_(ij) where a subscript i is indicative of antenna assembly and asubscript j is indicative of tag identity.

In operation, the device 500 uses the antennae assemblies TX1, TX2, TX3in sequence to interrogate the tags 10 a, 10 b. The assemblies TX1, TX2,TX3 are individually sequentially selected by way of the S_(ANT) signalcontrolling the switch unit 520; for example, the assembly TX1 isselected when the S_(ANT) signal sets the switch unit 520 to connect itswiper terminal d to the terminal a. It can be seen from FIG. 9 that eachof the tags 10 a, 10 b cannot be equidistant from the antennaeassemblies TX1, TX2, TX3 when the assemblies are arranged substantiallyco-linearly. As a consequence of such inequality in distance,heterodyned reflected components from the tags 10 a, 10 b as illustratedin FIG. 8 are moved along the abscissa axis 400 as the baseband receiver280 selects through the antennae assemblies TX1, TX2, TX3. Thus, wherefrequency overlap of components 420, 430 arises for one of the antennaeassemblies, the components are resolved in frequency when another of theantennae assemblies are used. Thus, the baseband receiver 280 is capableof isolating heterodyned components for signal processing purposes wherecontention arises on account of two or more tags being equidistant fromone or more of the antennae assemblies.

If required, device 500 can be further modified so that it uses one ofmore of the antennae assemblies TX1, TX2, TX3 to monitor the tags 10 a,10 b at intervals as they are moved along the conveyor belt 510; as thetags 10 a, 10 b are moved along, their relative distances from one ormore of the antennae assemblies TX1, TX2, TX3 changes thereby moving thecomponents 420, 430 around along the abscissa axis 400. As a consequenceof such movement, contention between the tags 10 a, 10 b at one instantare resolvable at another instant when the tags 10 a, 10 b move alongthe conveyor belt 510.

In a third approach to addressing contention, the devices 200, 500 andthe microcontrollers of the tags 10 a, 10 b are modified. The devices200, 500 are modified so that each incorporates a connection from itsrespective baseband receiver 280 to its radio frequency oscillator 220for inserting synchronisation pulses at intervals into the signalS_(RF). The microcontrollers in the tags 10 a, 10 b are also modified sothat, in operation, they monitor the potential difference P appliedthereto for the presence of the aforesaid synchronisation pulses. If oneor more of the tags 10 a, 10 b do not repetitively detect thesynchronisation pulses within a detection time window after outputtingtheir signature code to modulate their potential difference P, the oneor more tags 10 a, 10 b switch themselves for a period of time into aninactive state where they refrain from outputting their signature code.The devices 200, 500 are arranged to output synchronisation pulses at aperiodic interval related to the clocking rate of a particular code ofinterest each time after the devices 200, 500 detect the presence of thesignature code in the signal S_(BB). By adopting this approach, thedevices 200, 500 are capable of signaling to a preferred tag 10 a, 10 bto instruct it to keep outputting its signature code and force the othertag 10 a, 10 b into a waiting state where it is temporarily hinderedfrom outputting its signature code. The approach can, of course, also beemployed where there are more than two tags present.

A method of interrogation associated with the third approach using thedevice 200 to interrogate the tags 10 a, 10 b and select the tag 10 a inpreference to the tag 10 b will now be further elucidated with referenceto FIGS. 2 and 7:

-   STEP 1: The device 200 is instructed from a tag management system    (not shown) connected thereto that the tag 10 a is specifically to    be identified and other tags present are to be forced into the    waiting state;-   STEP 2: The device 200 commences by generating the signal S_(RF),    amplifying the signal S_(RF) to provide the amplified signal AS_(RF)    for outputting at its transmitter antenna 240 as the radiation 70.    The radiation 70 propagates to the tags 10 a, 10 b whereat it is    received. The radiation 70 is converted to the signal S1 in each of    the tags 10 a, 10 b and rectified to generate the potential    difference P therein. The microcontrollers 60 in each of the tags 10    a, 10 b are energised by the potential difference P and proceed to    apply a fluctuating load to their potential difference P in a manner    related to the signature codes of the tags 10 a, 10 b. The    fluctuating load causes a correspondingly fluctuating impedance to    be presented to the antenna assemblies 20 of the tags 10 a, 10 b    thereby temporally modifying their reflection characteristic and    hence giving rise to modulated reflected components 300, 310 of the    radiation 70 from the tags 10 a, 10 b bearing the signature codes.    The reflected components 300, 310 propagate back to the receiver    antenna 250 of the device 200 whereat they are received;-   STEP 3: The radiation components 300, 310 are converted at the    receiver antenna 250 to the corresponding signal S_(TF). The    amplifier 260 amplifies the signal S_(TF) to generate the    corresponding amplified signal AS_(TF). The amplified signal AS_(TF)    is heterodyned with the signal S_(RF) to generate the baseband    signal S_(BB) which is then passed to the baseband receiver 280;-   STEP 4: The receiver 280 filters the signal S_(BB) to isolate    spectral components therein and then detects their temporal    amplitude fluctuations to identify signature codes present in the    signal S_(BB). The receiver 280 selects the identified signature    code correlating with that communicated to the device 200 from the    aforementioned tag management system. At a response time interval    after identifying the correlating signature code, the interval    governed by the clocking rate of the correlating signature code as    measured by the receiver 280, the receiver 280 issues a pulse    command to the oscillator 220 which inserts a pulse into the signal    S_(RF);-   STEP 5: The signal S_(RF) including the inserted pulse is amplified    by the amplifier 230 to generate the signal AS_(RF) which is emitted    from the transmitter antenna 240 as the radiation 70;

STEP 6: The tags 10 a, 10 b monitor the radiation 70 received thereatfor the inserted pulse; if the pulse occurs within a time-window afterthe tags 10 a, 10 b have output their respective signature code in STEP2, the tags 10 a, 10 b will thereby identify whether they are to remainactive or temporally switch to the waiting state; in this example, thepulse occurs in the expected time-window of the tag 10 a and not in thatof the tag 10 b thereby forcing tag 10 b into the waiting state and thetag 10 a to remain active. As the tag 10 b switches to the waitingstate, contention between the tags 10 a, 10 b is thereby avoided;

-   STEP 7: The device 200 repetitively outputs synchronisation pulses    in the radiation 70 as described above in STEPS 4 and 5 until the    device 200 is instructed from the tag management system to search    for an alternative tag. Thus, the tag 10 b returns from the waiting    state to an active state where it can be selected after a recovery    time period, the time period being, for example, in a range of    microseconds to one second.

The inventors have appreciated that the tag 10 can be modified toprovide it with an extended operating range when powered from radiationreceived thereat. FIG. 10 provides a circuit of such a modified tagindicated generally by 600. Compared to the tag 10, the tag 600additionally includes a piezo-electric transformer indicated by 610 andan output diode detection unit indicated by 650 shown included within adashed frame 660. In the tag 600, the transformer 610 comprises aprimary region 620 and a secondary region 630 integrated to form aunitary elongate mechanical component. The regions are fabricated from apiezo-electric composition such as lead zirconate titanate (PZT) orsimilar piezo-electric material. The component has the primary region620 at a first end thereof and the secondary region 630 at a second endthereof. The component, in operation, is capable of being excited intoan elongate mode of resonance. On account of the primary region 620comprising a stack of electrically parallel-connected piezo-electricplates and the secondary region 630 comprising a single slab ofpiezo-electric material, the transformer 610 is capable of increasingthe voltage amplitude of a signal output from the diode detection unit16 applied to the primary region 620 to generate an enhanced potentialdifference P at the output of the diode detection unit 650 for poweringthe logic unit 20 and the voltage controlled oscillator 18.

Preferably, the transformer 610 is designed to resonate in its elongatemode in a frequency range of 10 kHz to 300 kHz, the resonance havingassociated therewith a Q-factor in the order of 20 to 500. For example,the transformer 610 can be manufactured so that:

-   (a) the primary region 620 comprises a stack of 5 to 20 layers of    polarised piezo-electric material, each layer having a thickness in    a range of 50 :m to 0.2 mm and major surfaces each being of an area    in a range of 1 mm×1 mm to 5 mm×5 mm; the stack can be formed by    adhesively bonding or soldering the layers together at these major    surfaces; moreover, the major surfaces can be metallized for making    electrical connection thereto; and-   (b) the secondary region 630 can comprise a slab of polarised    piezo-electric material having major front and rear major surfaces    each having an area in a range of 1 mm×1 mm to 5 mm×5 mm and a    thickness in a range of 0.3 mm to 1 mm.

The primary and secondary regions 620, 630 can be adhesively bonded orsoldered together to form a unitary structure.

The detection unit 650 includes a Schottky diode pair indicated by 670and an associated bypass capacitor 680 connected together in a similarmanner to the detection unit 16 as shown in FIG. 10. Primary andsecondary terminals T₁, T₂ of the primary region 620 are connected tofirst and second electrodes of the bypass capacitor 28 respectively. Asecondary region 630 terminal T_(S) is connected to a mid-point of thediode pair 670 as shown in FIG. 10.

Operation of the tag 600 in combination with the interrogating device200 shown in FIG. 7 will now be described.

The generator 220 of the device 200 generates a radio frequency signalat a frequency f₁ appropriate for the tag 600 and amplitude modulatesthe radio signal at a frequency f₂ corresponding to the elongateresonant mode of the transformer 610 at which the transformer 610 iscapable of providing a potential increase from its primary region 620 toits secondary region 630 as elucidated in the foregoing. The poweramplifier 230 amplifies the amplitude modulated signal and the resultingamplified signal is output from the antenna assembly 240 wherefrom it isradiated as the radiation 22. The radiation 22 is received at theantenna assembly 12 of the tag 600 whereat it causes a correspondingreceived signal S_(R) to be generated across the first and secondterminals of the assembly 12. The received signal S_(R) passes to thediode detection unit 16 which rectifies the signal S_(R) to produce acorresponding unipolar signal S_(T) which comprises signal componentspredominantly at the frequency f₂. The signal S_(T) excites thetransformer 610 into resonance along its elongate axis to generate asignal S_(S) at the secondary region of the transformer 650 at thefrequency f₂, the signal S_(S) having a greater voltage amplitudecompared to the signal S_(T). The signal S_(S) passes to the diodedetection unit 650 which rectifies it to provide the potentialdifference P for powering the logic unit 20 and the oscillator 18 asdescribed in the foregoing, the oscillator 18 providing the clock signalCLK at a frequency which increases as P increases.

The transformer 610 is particularly appropriate for use in the tag 600;wire-wound ferrite-cored or air-cored transformers are more bulky and donot easily provide an appropriate range of terminal impedances comparedto the transformer 610. Moreover, switched capacitor-type voltagetransformers are also not appropriate because they require anappreciable voltage to function.

The inventor has appreciated that the piezo-electric transformer 610 canhave a relatively high mechanical O-factor at resonance, for exampleoften exceeding several hundred in value, especially if a hard PZTceramic material exhibiting a dielectric loss of 0.005 or less at a testfrequency of 1 kHz is utilized. Such a hard PZT material is, forexample, available from a Danish company Ferroperm A/S, Hejreskovvej18A, DK-3490 Kvistgaard, Denmark under product reference PZT26. Thedielectric loss of a piezo-electric material is defined as the tangentof the electrical loss angle observed when electrically driving thematerial. The dielectric loss also represents the ratio of resistance toreactance of a parallel equivalent circuit of a piezo-electrictransformer made from the material. The dielectric loss can be measureddirectly using an impedance bridge, for example at an excitationfrequency of 1 kHz.

As a consequence, the transformer 610 provides a bandwidth limitingfilter for load modulation communicated from the microcontroller throughthe transformer 610 back to the antenna assembly 12. In order toincrease the modulation bandwidth of the tag 600, the inventor hasdevised a modified version of the tag 600 as indicated by 700 in FIG.11. The tag 700 is identical to the tag 600 except that the input/output(I/O) terminal is connected via a load resistor 710 to the secondelectrode of the capacitor 24 as shown. Thus, the tag 700 is capable ofapplying load modulation directly to the antenna assembly 12 and cantherefore respond at a greater data rate compared to the tag 600.

The inventor has appreciated that, although inclusion of the transformer610 and its associated diode detection unit 650 can provide an extendedtag operating range, inclusion of the transformer can introduce otherproblems. However, the resonant frequency of the transformer 610 changeswith temperature. In many situations, a tag interrogating device willnot have information regarding the temperature of the tags with which itis attempting to communicate. Thus, it is possible for the interrogatingdevice to select an inappropriate frequency f₂ which does not exactlycoincide with the resonant frequency of the transformer 610 in each ofthe tags. Such inexact coincidence is especially relevant where thetransformer 610 is selected to have an especially high Q-factor toprovide the tags 600, 700 with greatly extended range. In order toaddress such problems, the inventor has devised a solution whichinvolves the interrogating device sweeping the frequency f₂ of theradiation 22 in a cyclical manner. In the tags 600, 700, the potentialdifference P will vary as the frequency f₂ in the radiation 22 is sweptthrough resonance of the transformer. Thus, the rate at which the tags600, 700 output their respective signature codes will change in responseto the frequency f₂ being swept; in FIG. 8, such sweeping corresponds tothe components 420, 430 being swept along the abscissa axis 400. Theinterrogating device can be programmed to monitor movement of thecomponents 420, 430 as the frequency f₂ is swept and thereby determine afrequency f₂ appropriate for optimally operating each tag 600, 700.Moreover, by deliberately arranging for tags 600, 700 to have mutuallydifferent transformer 610 resonant frequencies, for example bydeliberately relaxing their manufacturing tolerances, such a sweptfrequency approach can be used to assist with resolving contentionbetween tags; as the tags have mutually different frequencies, theirrespective components would reach a maximum correspondingright-hand-side position along the abscissa axis 400 at mutuallydifferent f₂ frequencies.

The inventor has also appreciated that tag interrogating devices can beused not only for determining whether or not a particular tag is presentbut also its angular bearing with respect to the interrogating device.When tags and their associated interrogating devices are operated atrelatively low frequencies, for example in a range of 100 kHz to 200kHz, coupling between the devices and the tags arises predominantly byway of magnetic H-field coupling. Moreover, loop antennae are preferredat such a low frequency range, for example in manner like ferrite coilaerials in long-wave radio receivers which are direction sensitive.Thus, the inventor has devised an interrogating system and associatedcompatible tag capable of tag direction measurement, the systemindicated by 800 and the tag indicated by 810.

The system 800 includes an interrogating device 830 coupled to anantenna assembly indicated by 820. The assembly 820 comprises first andsecond loop antennae 822 a, 822 b configured mutually orthogonally asshown. The tag 810 is similar the tag 10 illustrated in FIGS. 1 and 2except that the antenna assembly 12 is implemented in the form of a loopantenna 840. The system 800 is arranged in a similar configuration tothe device 500 in that the switch unit 520 is arranged to selectivelyswitch between the loop antennae 822 a, 822 b.

The magnitude of the potential difference P developed in the tag 810,for a given angular of the tag 810 relative to the system 800 will be,to a first approximation, proportional to cos N and sin N for the firstand second loop antennae 822 a, 822 b respectively. As elucidated in theforegoing, the potential difference P determines the rate at which thetag 810 outputs its signature code. The rate of signature code output isdependent upon the potential difference P and is included as informationin reflected radiation from the tag 810, the information manifest as thefrequency of signature code components in the reflected radiation. Thereflected radiation received at the interrogating device 830 isprocessed therein to isolate the components corresponding to the tag 810for the two antenna 822 a, 822 b and their relative frequency shift, forexample along the abscissa axis 400 in FIG. 8, determined. The relativefrequency shift provides an indication of the ratio of sin N to cos Nand hence an indication of tan N. By applying an arctan calculation, theangle N can, at least to a first order of magnitude, be determined andhence the bearing of the tag 810 relative to the system 800 established.

It will be appreciated by one skilled in the art of tag design that thetags 10, 50, 60, 80, to 100, 600, 700 can be adapted for receivingradiation at other frequencies. For example, the antenna assembly 12 ofthe tag 10 can be replaced by a piezo-electric ultrasonic transducer sothat the tag can be interrogated and powered using ultrasonic radiation,for example ultrasonic radiation in a frequency range of 20 kHz to 500kHz; inclusion of such ultrasonic transducers enables the tag 10 to beused in marine applications, for example in the off-shore petroleumindustry. Alternatively, the antenna assembly 12 of the tags 10, 50, 60,80, 100, 600, 700 can, for example, be replaced with a photodetector forreceiving interrogating optical radiation and generating therefrom tagoperating power; tag return radiation can be provided using a pulsedlight emitting diode (LED) source or an actuated micromirror. Opticalradiation in the context of the present invention is intended to meanelectromagnetic radiation having a free-space wavelength in a range of10:m to 100 nm.

It will further be appreciated that the voltage controlled oscillator 18as illustrated in FIGS. 2, 3, 4, 5, 6, 10 and 11 can be implemented inother ways. For example, the oscillator 18 can alternatively employ afixed frequency oscillator outputting in operation to a digital dividerwhich provides the clock signal CLK at its frequency divided output, thedivision factor being controlled by the potential difference P by way ofone or more voltage comparators and associated logic gates forcontrolling the divider. For example, the tags 10, 600 can be modifiedsuch that their oscillator 18 is implemented as a fixed frequency 13.56MHz oscillator conforming to an international standard frequencypresently being established for tags, for example as employed in thePhilips Icode integrated circuit. The fixed frequency oscillator outputcan be preferably selectively divided in response to changes in thepotential difference P as provided in Table 1:

TABLE 1 Selected dividing ratio after the Potential 13.56 MHz fixedfrequency Frequency of the CLK difference, P oscillator clocking signal1.8 volts 256 52.96875 kHz 2.0 volts 128 105.9375 kHz 2.2 volts 64211.875 kHz 2.4 volts 8 1.695 MHz 2.6 volts 2 6.789 MHz

When the incoming radiation 22 is at a frequency of 13.56 MHz, thesignal S1 can be used directly to provide the fixed frequency oscillatoroutput signal for dividing down according to Table 1 to clock themicrocontroller in the logic unit 20. Such an approach results in aplurality of the tags 10, 50, 60, 80, 100, 600, 700 thus modifiedoperating synchronously when simultaneously interrogated; thissynchronous operation eases signal processing tasks within aninterrogating device used and the modified tags.

It will be further appreciated that the tags 10, 50, 60, 80, 100, 600,700 can be operated so close to the interrogating devices 200, 500 thatthe voltage controller oscillator 18 and the logic unit 20 can bedamaged by excessive potential difference P. In order to circumvent suchdamage, the inventor has appreciated that one or more voltage limitingcomponents can be included within the tags 10, 50, 60, 80, 100, 600,700. One convenient manner to include a voltage limiting component inthe tag 10 is to connect a Zener diode across the bypass capacitor 28,cathode and anode regions of the Zener diode being connected to thefirst and second electrodes respectively of the capacitor 28. Similarly,one convenient manner to include a voltage limiting component in thetags 600, 700 is to connect a Zener diode across the capacitor 680,cathode and anode regions of the Zener diode being connected VDD and VSSterminals of the logic unit 20's microcontroller.

1. A method of interrogating a variable frequency tag, the methodcomprising: emitting, by an interrogating device, interrogatingradiation toward the variable frequency tag; receiving, by theinterrogating device, response radiation from the variable frequency tagand generating a corresponding received signal, wherein the responseradiation includes a signature code that identifies the variablefrequency tag, and wherein the signature code is received from the tagat a rate dependent on a supply potential difference energizing the tag;and filtering, by the interrogating device, the received signal toisolate at least one spectral component corresponding to the variablefrequency tag, extracting the signature code of the variable frequencytag from the at least one spectral component, and correlating thesignature code with at least one signature template to identify thevariable frequency tag.
 2. The method of claim 1, wherein theinterrogating radiation comprises a component for exciting a transformerof the variable frequency tag into vibration, the method furthercomprising sweeping the interrogating radiation in frequency fordetermining when the variable frequency tag is operating at resonance ofits transformer.
 3. A method of interrogating a variable frequency tag,the method comprising: receiving, by the variable frequency tag,interrogating radiation emitted by an interrogating device andgenerating a corresponding received signal; generating a signaturesignal using the received signal, wherein the signature signal includesa signature code that identifies the variable frequency tag, and whereinthe signature code is output at a rate dependent upon a supply potentialdifference energizing the variable frequency tag; and radiating thesignature signal as response radiation from the variable frequency tag.4. The method of claim 3, further comprising deriving the supplypotential difference from the received signal.
 5. The method of claim 4,further comprising enhancing the supply potential difference using apiezo-electric step-up transformer.
 6. The method of claim 5, whereinthe interrogating radiation comprises a component configured to excitethe transformer into vibration.
 7. A variable frequency tag, comprising:an interface configured to receive interrogating radiation and toconvert the interrogating radiation into a received signal having amagnitude, wherein the interface is further configured to radiate aresponse signal; a processor configured to process the received signaland to modulate the response signal with a signature code usable toidentify the variable frequency tag; a clock configured to control arate at which the signature code is output to modulate the responsesignal, wherein the rate is a function of the magnitude of the receivedsignal; and a power supply providing an electric potential sufficient toenergize the tag.
 8. The variable frequency tag of claim 7, wherein theinterface includes a piezo-electric ultrasonic transducer for receivingultrasonic radiation.
 9. The variable frequency tag of claim 7, whereinthe interface includes a photodetector for receiving interrogatingoptical radiation and generating therefrom operating power for thevariable frequency tag.
 10. The variable frequency tag of claim 9,wherein the interface includes a pulsed light emitting diode.
 11. Thevariable frequency tag of claim 9, wherein the interface includes anactuated micromirror.
 12. The variable frequency tag of claim 7, whereinthe interface includes an antenna assembly comprised of a folded dipoleantenna.
 13. The variable frequency tag of claim 7, wherein theinterface includes an antenna assembly that is coupled via a matchingnetwork to a detection assembly.
 14. The variable frequency tag of claim13, wherein the detection assembly includes a dual-zero-bias Schottkydiode detector.
 15. The variable frequency tag of claim 14, wherein thedual-zero-bias Schottky diode detector comprises a pair of seriallyconnected Schottky diodes and a bypass capacitor.
 16. The variablefrequency tag of claim 7, wherein the clock includes a fixed frequencyoscillator coupled to a digital divider, wherein the digital divider isconfigured to receive a potential difference, and wherein the digitaldivider is further configured to divide the output of the fixedfrequency oscillator by a division factor based on the potentialdifference to generate a clock signal.
 17. The variable frequency tag ofclaim 7, wherein the clock includes a voltage-controlled oscillatorconfigured to receive a substantially unipolar, fluctuating potentialdifference.
 18. The variable frequency tag of claim 17, wherein a clocksignal output by the voltage-controlled oscillator has a frequency thatvaries based on the received potential difference.
 19. The variablefrequency tag of claim 17, wherein the voltage-controlled oscillatorcomprises a ring-of-three oscillator that includes a series of a firstinverting logic gate, a second inverting logic gate, and a thirdinverting logic gate connected in a feedback loop configuration.
 20. Thevariable frequency tag of claim 13, wherein an output of the processoris coupled to the detection assembly and is configured to present atemporally varying load to the detection assembly for influencing aproportion of radiation reflected by the antenna assembly.
 21. Thevariable frequency tag of claim 20, wherein the output of the processoris coupled to a bypass capacitor of the detection assembly.
 22. Thevariable frequency tag of claim 20, wherein the output of the processoris coupled to a blocking capacitor via a load resistor, wherein a firstelectrode of the blocking capacitor is coupled to the antenna assembly,wherein a second electrode of the blocking capacitor is coupled to thedetection assembly, and wherein the output of the processor is coupledto the second electrode of the blocking capacitor.
 23. The variablefrequency tag of claim 7, wherein an output of the processor is coupledto the clock and is configured to modulate a frequency at which theclock oscillates based on the signature code.
 24. The variable frequencytag of claim 7, wherein the interface includes an antenna assembly andan auxiliary antenna, wherein the antenna assembly is communicativelycoupled to the auxiliary antenna, and wherein the auxiliary antenna andthe antenna assembly are tuned to different radiation frequencies. 25.The variable frequency tag of claim 24, further comprising a surfaceacoustic wave oscillator, wherein the auxiliary antenna is configured toreceive a signal for emission from the surface acoustic wave oscillator.26. The variable frequency tag of claim 24, wherein an output of theprocessor is coupled to the auxiliary antenna and is configured tomodulate a proportion of radiation reflected by the auxiliary antenna.27. The variable frequency tag of claim 26, wherein the output of theprocessor is coupled to the auxiliary antenna via a bias resistor. 28.The variable frequency tag of claim 7, further comprising a logicoscillator configured to oscillate at a substantially constantfrequency.
 29. The variable frequency tag of claim 28, wherein a clockinput of the processor is configured to receive a constant clock signalfrom the logic oscillator, and wherein a gate input of the processor isconfigured to receive a variable clock signal from the clock.
 30. Thevariable frequency tag of claim 29, wherein the processor is configuredto repeatedly output the signature code at intervals based on thevariable clock signal at a frequency based on the constant clock signal.31. The variable frequency tag of claim 7, wherein the interfaceincludes an antenna assembly in communication with a detection assembly,the variable frequency tag further comprising a piezoelectrictransformer coupled to the detection assembly.
 32. The variablefrequency tag of claim 31, further comprising an output detectionassembly coupled to the piezoelectric transformer, wherein an output ofthe processor is coupled to the output detection assembly.
 33. Aninterrogating device for interrogating at least one variable frequencytag, the interrogating device comprising: a signal generator configuredto generate an interrogating signal; an interface configured to radiatethe interrogating signal to at least one variable frequency tag, and toreceive from the variable frequency tag a response signal that includesa signature code usable to identify the variable frequency tag, whereinthe response signal has been modulated with the signature code as afunction of a magnitude of the interrogating signal received by thevariable frequency tag; and a processor configured to filter theresponse signal and isolate a spectral component corresponding to thevariable frequency tag, and to extract the signature code from thespectral component to identify the variable frequency tag.
 34. Theinterrogating device of claim 33, wherein the processor is configuredto: detect more than one signature code within a given spectralcomponent at different times; and extract more than one signature codefrom the given spectral component to identify more than one variablefrequency tag equidistant from the interface.
 35. The interrogatingdevice of claim 33, wherein the interface includes a plurality ofantenna assemblies configured to selectively radiate the interrogatingsignal.
 36. The interrogating device of claim 35, wherein the pluralityof antenna assemblies are arranged substantially co-linearly, andwherein the processor is configured to extract more than one signaturecode from a reflected signal received by one of the plurality of antennaassemblies that is not equidistant from more than one variable frequencytag.
 37. The interrogating device of claim 35, wherein the interrogatingdevice is configured to interrogate more than one variable frequency tagwhile moving in relation to the more than one variable frequency tag.38. The interrogating device of claim 33, wherein the processor isconfigured to cause the signal generator to generate at least onesynchronization pulse for instructing a requested tag to output asignature code of the requested tag.
 39. The interrogating device ofclaim 33, wherein the interface includes a first loop antenna and asecond loop antenna arranged mutually orthogonally with the first loopantenna, wherein the processor is further configured to compare a signalreceived by the first loop antenna to a signal received by the secondloop antenna to determine a bearing of a tag relative to theinterrogating device.